High breakdown-voltage semiconductor devices are expected to have excellent radio-frequency characteristics and high breakdown voltage of source/drain regions.
In such a high breakdown-voltage semiconductor device, when a voltage higher than a certain level is applied to a drain region, a large electric field occurs at the boundary between the drain region and a channel region, and junction breakdown occurs at the boundary. Therefore, a problem in increasing breakdown voltage is how to reduce an electric field produced at the boundary between the drain region and the channel region.